156 ALASKAN GLACIER STUDIES
The southern portion of this frontal valley was an incomplete fosse, presumably of the same origin as that developed in 1905 when the glacier front stood much farther up the valley. The inner boundary of the fosse was the visible glacier front, the outer margin a sloping gravel terrace with a low, undulating slope toward the fosse and a gradual descent of its surface toward Seal Bay. The apex of the terrace was toward the large south stream and was evidently an alluvial fan of that stream, probably built in the spring. Between the southern edge of the fosse and the glacier there was a low morainic mass which was slumping rapidly. This extended northward along almost the entire glacier front, but grew lower and had less pronounced form toward the north. Throughout its entire length this hummocky area had ice beneath it, and the hummocks rose from 5 to 25 feet.
The alluvial fan terrace just described had numerous small, dry kettles, and just in front of the ice, and for some distance northward there were also kettles, some containing water. Ice was revealed in the southern side of the pitted alluvial fan where it was cut into by a swing of the large glacier stream. That it was an alluvial fan, built on the outer edge of the glacier during the period of spring melting was indicated by the presence of numerous channelways upon it, pointing toward the apex of the fan and, therefore, toward the south glacier stream which built it. From the conditions on and in this fan we find complete verification of the explanation previously given of the pitted outwash gravel plain which existed farther up the valley in 1905. In that year the process of burial of the glacier terminus was not so clearly demonstrable as in 1909, but of the 1909 condition there can be no doubt as to the cause. The steep slope of the alluvial fan toward the north and west, and the presence of numerous channelways extending in the same direction, both pointed clearly to the source of the gravel-bearing water; and the discovery of ice beneath the pitted gravels verified by observation the inference previously drawn from the presence of the kettles, that they were the result of subsidence through melting out of buried ice. In the 1909 condition the aggradation by which the ice front was buried beneath the gravels was far less extensive than in 1905, and the area occupied was much smaller, though otherwise the phenomena were essentially the same.
Although not tidal in 1909, the Hidden Glacier discharged ice into Russell Fiord. Several scores of these icebergs were afloat in Seal Bay in July, 1909, some of them three or four feet in diameter. They had floated down the largest glacial stream from the glacier front.
A comparison of maps and photographs of the coast of Seal Bay in 1905 and 1909 shows that the northern side of the outwash plain, which projected most in 1905, had grown almost none in connection with this two mile advance of the glacier, while the south side, which projected most in 1909, had grown forward 900 or 1000 feet since 1905. The southern side is the side where the largest glacial stream entered the bay between 1905 and 1909. There are two photographs showing this coast from exactly the same site in 1905 and 1909, and exhibiting this delta advance graphically, though unfortunately they cannot be compared exactly because we do not know the stages of tide at which they were taken.
Interpretation of the Advance. The condition of Hidden Glacier in 1909, as contrasted with its condition in 1906, proves a very great advance and breaking of the glacier during the interval of three years. In view of the resemblance of the phenomena of the Hidden Glacier to those observed in other glaciers of the region and of the brief interval of time